The invention relates to rotary viscosimeters in which a test sample is placed into a cylindrical measuring cup. A rotary measuring cylinder extends into the cup so that the sample is disposed in a gap between the cylinder and the cylindrical wall of the cup. Forces required to rotate or angularly deflect the cylinder are measured and used to determine the viscosity of the sample.
FIG. 1 schematically illustrates a known rotary viscosimeter which employs a rotary cylinder and which forms the basis for the present invention. Such a rotary viscosimeter uses a measuring motor 1 which rotates a measuring cylinder 7 via a shaft 4. The relationship between the torque on the motor shaft and its electrical supply, particularly its current consumption, frequency or phase shift, are known. The torque generated by a test sample 6 can therefore be determined from a parameter of the electric supply for the motor. In addition, an angle sensor 2 is provided for determining the angular rotational position of shaft 4 or the number of its rotations. Also important is a bearing for journalling shaft 4. Depending on the constructional details and the required torques, roller bearings or, as in the illustrated case, air bearings 3 can be used.
Generally speaking, three different measuring systems with standardized geometry are in use, and they include cone/plate measuring systems, plate/plate measuring systems and, as shown in FIG. 1, cylinder measuring systems.
Such a rotary viscosimeter further has a relatively rigid support 8 for measuring motor 1 and bearings 3. The support holds a measuring cup 5 which receives sample 6, measuring cylinder 7 and, if applicable, a temperature control system for maintaining a constant sample temperature.
For determining the parameters of the sample, the torque can be measured by rotating shaft 4 at a constant rpm (CSR test). It is also possible to apply a constant torque to shaft 4 and to measure the rpm or rotational deflection of the shaft (CSS test). Finally, shaft 4 can be subjected to a sinusoidally or otherwise oscillating rotational movement (oscillation test). This last testing method permits one to determine the elastic component of sample 6 in addition to its viscosity.
Samples 6 can be liquids, gels, pastes, melts, as well as granulates or powder made from solid bodies. The viscosity of such samples is highly dependent on the temperature, which frequently can change as much as 10% per 1.degree. C. An accurate determination of the viscosity therefor requires a homogeneous temperature distribution within the sample, particularly the portion thereof in measuring gap 10. Since the viscosity of many samples, such as heat curing cements and resins, also changes with time, it is desirable to heat or cool the sample and attain a uniform sample temperature in as short a time as possible.
FIG. 2 schematically illustrates a coaxial cylinder measuring system. Sample 6 that is to be measured is in a gap 10 between the inner surface of a stationary measuring cup 6 having a radius "R" and measuring cylinder 7 which has a radius "r", a height "h", and is driven by motor 1. The sample completely surrounds the measuring cylinder. Upon rotation of measuring cylinder 7, the sample in gap 10 is sheared and its viscosity can be determined from the rpm, the torque, and the geometry of the gap. An accurate determination of the viscosity requires a uniform temperature distribution in the sample, particularly in the area of gap 10, which has a thickness "s". In addition, the temperature of the sample must be accurately measured. Since a temperature sensor placed inside gap 10 would affect the shearing of the sample, it is necessary to measure the temperature outside the gap but as closely as possible to the sample. A convenient location for placing temperature sensor 3 is the wall of measuring cup 5.